It is becoming increasingly popular to monitor a physiological parameter of a subject using an apparatus including a wearable device. Such devices are convenient to use as they offer increased freedom of movement for the user whilst a physiological parameter is being monitored. In this way, it is possible to measure physiological parameters in a variety of circumstances, for example at different levels of physical exertion of the subject.
The physiological parameter sensor may be mounted to the user differently in different applications. The position of the mounted physiological parameter sensor relative to the subject when the sensor may vary according to the parameter to be measured, the type of physiological parameter sensor, and/or the circumstances in which physiological sensing takes place. In some cases, the physiological parameter sensor should be in contact with the user's body. In other cases, the physiological parameter sensor should be separated from the subject's body. An important problem with respect to wearable sensors is ensuring that the contact pressure or separation between the physiological parameter sensor and the user's body is maintained at a constant level since the signal obtained during physiological sensing is affected by the relative position of the wearable sensor and the user. For instance, in PPG monitoring, a steady distance between the light sensor and the skin is desired for optimal stability of the sensor signal. For ultrasound transducer patches, good contact with the skin is imperative for high quality images. Similarly, the electrodes of ECG monitoring devices are sensitive to contact pressure.
Therefore, one problem that can occur during monitoring of a physiological parameter using an apparatus including a wearable sensor is that the position of the sensor relative to the body of the user may change as a consequence of movement of the user, resulting in a change in the measured data. Movement of the user resulting in a change in the relative position of the wearable sensor and the user causes motion artefacts to be present in the measured data. While motion artefacts can be filtered out digitally, this approach can negatively impact the quality of the measurement data obtained using the wearable sensor.
There is therefore a need for a wearable device for measuring a physiological parameter that can be operated to obtain high quality data.